Ionisation gauge



July 31, 1956 G. K. T. CONN ETAL 2,757,306

IoNIsATIoN GAUGE Filed Feb. 23, .1955

United States Patent O 10N ISATION GAUGE George Keith Thurburn Conn, Sheffield, ,and Hugh Neville Daglish, Rochdale, England, assignors to National Research Development Corporation, London, England, a British corporation Application February 23, I1955, Serial No. 490,104 5 Claims. (Cl. 313-156) 'Ille present invention relates to ionisation gauges of the kind in which electrons are liberated in a region of low gaseous Pressure for the purpose of effecting :ionisation of the residual gas by collision of said electrons with the gas molecules, the ions liberated being drawn -oif by means of polarised electrodes, the ygas pressure being indicated by the magnitude of the .current associated with the withdrawal of the ions. j

Ionisation gauges of the above kind fall mainly .into two groups, namely those in which there is provided a .j cated cathode, an anode and a collector electrode ,arranged somewhat in the manner of an ordinary triode valve, and those in which a cold cathode is employed in conjunction with a collector electrode and a magnetic field.

In the first of these types, reliance is had on 'the thermionic emission from the cathode to supply electrons for ionisation and on the anode potential to `provide adequate acceleration of the electrons to the velocities necessary to effect ionisation. In the second group, release of electrons is brought about by the cathode/anode voltage, and the paths of the electrons between the cathode and anode are extended by the effect of the magnetic field which causes the electrons ,to execute Ygyrations, thus increasing the chance of collisions with `gas particles which produce ionisation.

The ionisation gauge to which the present invention relates may include features of both of these types I,and the invention has for one of its objects to provide an ionisation gauge of increased sensitivity in relation Vto known forms of gauge.

Another object of the invention is to provide an ionisation gauge for use in gas pressures of the order of 0.01 mm. of mercury or less.

The sensitivity of an ionisation gauge is related to the ion current which flows at different ,gas pressures and will depend upon the electron current, so that sensitivity may be measured as micro-amperes ion current per inicron pressure per milliamp. electron current.

It has been found that the geometrical arrangement of the electrodes in an ionisation gauge plays an important part in the sensitivity of the device. Clearly it is desirable to achieve as high an ion current as possible for very low gas pressures. Clearly also it is desirable that the electron current required to achieve this result should not be too high. To meet these two requirements, it is desirable that the electrons, by which ionisation is brought about, should travel the greatest possible distance within the electrode structure before reaching the anode; also it is important that the greatest possible proportion of ions liberated by electron collisions should be drawn oli?. These considerations have led to the design of an ionisation gauge to which this invention relates.

The gauge according to the invention therefore comprises a thermionic cathode of lamentary form, an anode arranged so as to set up a field tending to draw electrons liberated from the cathode radially away from the cathde, and collector electrodes disposed at both ends of ICS Ithe cathode structure so as to provide electron-repellant fields having a component extending longitudinally paralle'l to the cathode. The electrode structure may comprise a cylindrical anode surrounding the cathode and the collector electrodes may be disc-shaped and may at least partially cover the open ends of the anode cylinder; the electrodes may be of plate, of Wire or of gauze, the performance differing slightly in the various cases.

In addition -to the above, there may be provided means `for setting lup a magnetic field extending axially in relation to the cathode filament which will have the effect of causing the electrons toy spiral around the cathode filament.

In order that the invention may be more clearly unders'tood, an embodiment thereof will now be described with reference lto Athe accompanying drawings, in which Fig. l shows the electrode structure of an ionisation gauge according to the invention and Figs. 2 to 5 are diagrams illustrating operating characteristics of gauges according tothe invention. Y

In Fig. l there is shown Va iilamentary cathode 1,

which Amay be of tungsten wire, stretched between two supports 2 and 3, of which one may be rigid and the other springy, so as to maintain the filament properly tensioned. Coaxial Vwith the filament is a sheet-metal cylindrical anode 4 supported on a suitable support wire 5. The two ends of the anode are covered by discshaped collector electrodes 6 and 7, supported on the two arms of a Y-shaped wire support 8. The discs 6l and 7 are spaced axially from the ends of the anode sufficiently far for purposes of insulation and they are provided at their centres with holes 9 for the passage of the lilament 1 ltherethrough with adequate clearance.

The `whole structure therefore forms a substantially closed box and is supported in a glass envelope 10 in conventional manner on the lead wires which are Sealed through the envelope. The envelope 10 has outlets at 11 and 12 which may be connected to pumping apparatus and to the evacuated chamber, the pressure in which is to 'be monitored by the gauge.

With such a structure, a high order of sensitivity can "be achieved. In one example, an ion current of 45 micro-amps. per micron at l milliamp. electron current was measured.

The sensitivity of the device can, however, be enormously increased by immersing the electrode structure v in a magnetic field (which may be an A. C. or a D. C. field), `as suggested diagrammatically by the pole pieces 13 and 14, which set up a magnetic field extending axially yparallel to the filament. These may be the poles of a permanent magnet or of an electromagnet, which in the latter case enables .control of the magnetic field to be exercised. lf desired, of course, the magnet could be located inside the envelope 10. The effect of such a lield is to cause the electrons to execute spiral paths as they travel outwardly from the lament, and the magnetic lield may be adjusted in relation to the anode Voltage so that many gyrations are executed before the electrons reach the anode. The effect of the negatively polarised collector electrodes 6 and 7, moreover, is to cause the electrons to travel to and fro axially of the electrode structure, and the result is a complex electron path. This greatly increases the number of collisions between electrons and ions to be expected, even at very low gas pressures, so that increased ion currents can be achieved with relatively low electron currents at low gas pressures.

The behaviour of the device will obviously depend on the following factors:

(l) Filament temperature (2) The anode voltage 11.1! (3) The magnetic field (4) The collector voltage Electrode geometry The filament temperature provides a direct control of the electron emission, and the sensitivity of the device is improved in proportion with the increase in emission. Normally, however, the temperature of the ilament will be chosen so as to provide a reasonable filament life.

The anode voltage and the magnetic field are interrelated and, in a particular example, the anode was run at 400 volts and the magnetic iield was 163 oersteds. These two parameters will normally be fixed so that the magnetic eld is at or slightly above that which would normally produce cut-oit of anode current.

The collector voltage is negative to that of the cathode and normally is in the range to -60 volts; in -a particular example, -54 volts. The value of this voltage will control the value of the magnetic tield at which the maximum ion current will be reached at a given anode voltage, Vand it is clearly possible to adjust the various parameters to obtain optimum operation of the device in any desired manner.

In Figures 2, 3, 4 and 5, there are shown typical characteristics for a gauge according to the invention operated under diierent conditions. Fig. 2 shows how the ion current (in fra.) varies with the collector voltage for different values of electron emission. The values of electron emission written against the curves are in ma. and all curves were taken with an anode voltage of 300 v. It will be seen that for low values of electron emission, the ion current is practically independent of collector voltage as the electron energy is low. However, as the electron emission is increased, a stage is reached when a nite voltage must be applied to the collector electrodes to prevent electrons swamping the ion current.

Fig. 3 shows the variation of ion current with collector voltage at various values of magnetic field, the electron emission being constant and the anode voltage iXed, in the example, at 225 v. The values of magnetic iield written against the curves are given in oersteds.

Fig. 4 shows the variation of ion current with magnetic eld for three diierent values of anode voltage. It will be seen from this iigure how the ion current reaches a well-dened maximum at a given magnetic eld, the maximum occurring at progressively higher magnetic iields as the anode voltage increases. Clearly it is possible to optimise the ion current by variation either of the magnetic eld or of the anode voltage.

Finally, Fig. 5 shows how the ion current varies with gas pressure in two conditions of operation. The dotted curve was taken with an electron emission current of 2 ma., a magnetic eld of 163 oersteds, anode voltage of 400 volts and collector voltage of -54 volts. This corresponds to a sensitivity of 7,000 ua. at 2 ma. per micron. The full line curve represents operation atan electron emission of 1 ma. with magnetic eld of 143 l oersteds, anode voltage of 225 volts and collector voltage of -54 volts. This corresponds to a sensitivity of 800 ya. at 1 ma. per micron.

Clearly the structure of the device may be varied from that illustrated and described above. For example, the anode may be of wire mesh, which may permit freer liow of gas particles through the electrode structure but may also have the etect that pumping due to ion extraction is increased, which may not at all times be desirable. The collector electrodes may, moreover, be of wire mesh and may vary in size and shape according to requirements, the important consideration being that they are located adjacent the ends of the anode and cover at least in part the annular spaces between the cathode and the open ends of the anode.

The materials to be employed for the electrodes may be conventional; e. g. tungsten for the lament and molybdenum for the anode and collector electrodes.

We claim:

1. An ionisation gauge comprising a tilamentary cathode electrode, at least two collector electrodes disposed one at each end of said cathode structure an anode electrode between said collector electrodes and encircling said cathode and an envelope enclosing said electrodes.

2. An ionisation gauge comprising a lamentary cathode electrode, a pair of collector electrodes disposed at opposite ends of said cathode, a cylindrical anode electrode between said collector electrodes and surrounding said tilamentary cathode, and an envelope enclosing said electrodes.

3. An ionisation gauge as claimed in claim 2 wherein said collector electrodes substantially close the space between said lilamentary cathode and the open ends of said cathode.

4. ionisation gauge as claimed in claim 2, wherein the collector electrodes are discs having central apertures through which said lamentary cathode passes and having a diameter greater than the cross-sectional diameter of the anode.

5. In an ionization gauge for measuring low gas pressures comprising an envelope communicating with a region of low gas pressure, within said envelope a lamentary cathode, a cylindrical anode surrounding said cathode and two collector electrodes located one at each end of said anode and spanning the space between said cathode and said anode, means for heating said cathode, means for polarizing said anode positively to said cathode and said collector electrodes negatively to said cathode, and means for setting up a magnetic field in a direction axially of said anode.

g References Cited in the ile of this patent UNITED STATES PATENTS 

